1. Field of the Invention.
The present invention relates to recessed fastener heads and, in particular, to a fastener head having a pair of recesses each recesses having a ledge formed below the head surface.
2. Description of the Prior art.
Most prior art internal wrenching recesses perform adequately when installation and removal torque loads are low and if the fastener material is at higher strength levels. Often at these higher strength levels the results of repeated usage shows some recess distortion and indications of recess material displacement. However, when the same recess is fabricated from a softer material that has high malleability characteristics, one torsional cycle often is sufficient to deform the recess configuration to cause the raised material (burrs) to appear on the fastener head surface. Repeated drive tool usage often results in camout and destruction of the recess making the fastener useless. Deformation of the recess usually begins at the junction of the recess and the surface head of the fastener, and displays itself by a deformed edge of raised material. This raised material is unsightly, can inflict some minor physical damage to persons who contact the area where the fastener is installed, and with regards to aerodynamic smoothness, is unacceptable.
All materials will deform to some extent when the load exceeds the yield point, but if the load is constant for all materials, it becomes readily obvious that softer materials will deform much more easily than harder materials. Currently available internal driving recesses do not take in account fastener material strength in order to minimize recess deformation and are configured identically the same for soft or hard materials. In other types of recesses, the installation tool is inserted into the mating internal wrenching recess. As torque is applied, contact between the tool's driving surface is resisted by the recess reacting surface. As torque increases and the fastener is seated, continuous torque could cause deformation to the upper surface of the recess. The reason for the deformation is the fact that this is the portion of the recess that has the weakest unsupported area and is susceptible to movement.
It is also noted that some prior art fastener recesses have drive angles ranging from fifteen to ninety degrees. The drive angle is the resultant radial force transmitted by the wrenching tool to the fastener material of the recess. As an example, the hexagon recess internal wrenching tool, commercially available, has six external corners. When these corners engage a mating hexagon recess, the applied torque load is divided between a torsional and radial direction. The drive angle for a hexagon recess is thus sixty degrees which exerts more loads radially rather than the desired tangential loading. This hexagon drive angle manifests itself by imposing very high radial forces to the recess, which in many instances encourages camout especially in softer materials. Recesses that have more than two driving surfaces have to deal with the problem of tolerancing and concentricity of the installation tool and the recess. Thus, if there is a slight discrepancy or difference in dimensions, angularity or concentricity, one portion of the driving tool will engage only one portion of the mating recess. This can contribute to premature tool breakage because the torque load is concentrated and unbalanced on one segment of the recess and causes it to deform.
An example of a prior art fastener which has a specific recess configuration is disclosed in U.S. Pat. No. 4,202,244 to Gutshall, which shows a head recess having a cruciform configuration and an annular groove or indent encircling about seventy-five percent of the periphery of the socket portion between each quarter portion of slots, the recess being designed to interact with a conventional cruciform driving tool or flat bladed screw driver. The recess described in this patent has the disadvantages noted hereinabove. A technique for preventing burrs from forming on the tool driving area of a fastener is disclosed in U.S. Pat. No. 3,969,974 to Lejdegard. Flats formed on the external wrenching surface allows displaced metal to build up on the flat instead of forming a burr on the outer cylinder surface.
What is therefore desired is to provide a new fastener recess which minimizes burrs on the surface of the fastener head, the recess drive angle being reduced to substantially zero degrees to minimize installation tool radial force loading and thus camout and wherein the number of driving surfaces is reduced to minimize problems of tolerance and concentricity.